Field of the Invention
The disclosed technology relates generally to patterning organic semiconductor layers, and more particularly to patterning organic semiconductor layers on a substrate, e.g., for fabricating organic photovoltaic modules with monolithically integrated photovoltaic cells.
Description of the Related Technology
Organic photovoltaic cells (OPV), e.g. photovoltaic cells fabricated via solution processing of organic inks such as polymer:fullerene inks, may provide high efficiencies while also providing relatively easy and low cost integration for diverse applications and large scale production. While small area cells can attain good power conversion efficiencies, larger area polymer devices sometimes suffer from efficiency-limiting losses. For example, efficiencies of larger devices may be limited due to low electrical conductivity of the transparent contact.
Low conductivity of the transparent electrodes may limit physical dimensions of individual organic photovoltaic cells. To mitigate the effects of the low conductivity of the transparent electrodes, several organic photovoltaic cells are sometimes connected in series to form a single module. In view of limiting the non-active zone between adjacent cells, it may be advantageous to provide monolithic integration of the cells on a single substrate.
The series connection of adjacent cells on a substrate using traditional methods are sometimes achieved by providing an unpatterned organic photoactive layer on a substrate comprising pre-patterned electrodes, followed by local removal of the organic photoactive layer in a subsequent process to define the individual cells. In this way, the limited conductivity of the transparent contact can be mitigated by combining several smaller cells in a series-connected chain. This approach can limit the produced photocurrent of the overall module, while increasing the voltage linearly with the number of cells connected in series.
The processing of these monolithically connected organic modules often includes patterning of each deposited layer in order to connect the cells in series. Spin coating, which may not be compatible with roll-to-roll processing, spray coating and doctor blading are coating techniques using which continuous layers, i.e. unpatterned layers, can be obtained. In order to fabricate modules from the unpatterned layers using traditional methods, additional patterning techniques such as mechanical scribing or laser scribing may be employed. These methods, however, apart from the extra processes and higher production costs, can have certain undesirable effects, such as increased risk of damage to the underlying layers and potentially the flexible foil that is commonly used as the holding substrate.
In addition, the organic photoactive layer may be patterned by a relatively fast technique, such as mechanical scribing or laser scribing. However, such techniques can create excessive debris, which may have certain additional undesirable effects, such as increased risk of electrical short-circuiting through the organic photoactive layer after deposition of top electrodes. Furthermore, when using mechanical scribing or laser scribing for patterning the photoactive layer, there may be additional undesirable effects such as increasing the risk of damaging the underlying substrate.
Techniques such as slot die coating can be used to form simple patterned lines on the substrate. In addition, printing methods such as screen printing, gravure printing and flexographic printing can be used to form more complex patterns, such as serial/parallel connections between single cells within the module. Some of these techniques, however, may require specialized pieces of equipment, as well as the adaptation of deposition conditions for small area devices to those for large area devices. These conditions, e.g. primarily drying, define the bulk morphology of the organic photovoltaic cell, e.g. of the polymer:fullerene film, and thus may be important for maximizing the power conversion efficiency of the final solar cell. In the following, a manufacturing process is disclosed, which may benefit from the advantages provided by spray coating and may allow for direct patterning of the photoactive film.
Furthermore, in “Effect of Self-assembled Monolayers on the Performance of Organic Photovoltaic Cells,” by H. Bedis, in Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, p. 42-50, the use of self-assembled monolayers (SAM) have been proposed to improve the carrier injection into an organic semiconductor, and to improve the adhesion of the organic material on a metal or oxide electrode and the photovoltaic performance of an organic cell.